1. Field of the Invention
The present invention relates generally to tuning apparatus for television receivers and the like, and more particularly to an improved tuning control system having electronic fine tuning capability.
2. Description of the Prior Art
The mechanical turret approach to television tuning has been used almost exclusively for the past 20 to 30 years. Even though replete with the inherent disadvantages of mechanical complexity, unreliability and cost, such apparatus has been technically capable of performing its intended function and as a result the consumer has had to bear the burdens associated with the device. However, with the recent FCC demands for parity of tuning for UHF and VHF channels, the increasing number of UHF and cable TV stations have imposed new tuning performance requirements which severely tax the capability of the mechanical turret tuner. Consequently, attempts are now being made to provide all electronic tuning to meet the new requirements.
One new tuning system currently being incorporated in some television receivers uses a varactor tuner which overcomes some of the disadvantages of mechanical turret tuner by accomplishing tuning electronically. As the name indicates, the heart of such a tuner is a varactor diode which is used as a capacitive tuning element in the RF and local oscillator sections. In this system, channel selection is made by applying a given reverse bias voltage to the varactor to change its electrical capacitance. The channel selection biasing can be performed by mechanically or electrically switching approximately 18 preset potentiometers. The problem with such arrangement is that it quite seriously limits the number of channels available to the consumer. Additionally, it suffers from the drawback that all potentiometers require adjusting for the desired channels. The VHF channels are usually factory adjusted while the six UHF channels require on-location adjustment. Moreover, using this arrangement, the only indication--during adjustment--of which channel is selected is by station identification.
However, since an electronic tuning system need accomplish only two basic functions, i.e., (1) the tuning and signal processing of the signal from antenna to the IF stage and (2) the control of the channel selection, entry and identification, the varactor tuner used in conjunction with the digital frequency synthesizer can provide 82 or more positively identified channels requiring no individual channel alignment. Where a varactor is ued as the tuning element, the control system must be capable of generating a DC voltage corresponding to the channel selected and must be capable of causing the channel selected to be displayed. Although digital phase-locked loop techniques, sometimes referred to loosely as frequency synthesis techniques, have been used in various communication receiver designs, it is only recently that they have been applied to television tuning. One of the reasons is that because of the frequency band allocations and the high frequencies used for UHF, it has been difficult to use such circuitry in television receivers. However, recent developments in high speed IC counters have made the approach much more practical for such applications.
The phase-locked approach offers several advantages. It is a closed loop system which self-compensates for varactor tolerances and component drifts; it is capable of operating with 82 or more channels; it provides easy interface with digital displays; it simplifies remote control design; it enables electronic channel selection to be provided; it is highly reliable; and it has no alignment problems.